Not many thinkers successfully straddle academia and politics, but one of the few who has managed to do so on both sides of the Atlantic is Robert Putnam, author of Bowling Alone. You can spot traces of his influence all over New Labour policy. He was the man who popularised the concept of social capital - the trust and networks of friendship, neighbourhood and organisations on which so much of our lives depend - and it has won him the ear of politicians of all persuasions: Bill Clinton, George Bush, Tony Blair, Gordon Brown, even, most recently, the Libyan leader, Muammar Gadafy.

Aware of how his work is used politically, Putnam is understandably nervous now about how he presents the first findings of the biggest study of social capital ever undertaken on which he has been working for over five years. He started out wanting to track social capital over time and in different communities across the US. What he wasn't expecting to find was a negative link between ethnic diversity and social capital. Put crudely, the more ethnically diverse the neighbourhood, the less likely you are to trust your local shopkeeper, regardless of his or her ethnicity. He warns that, however uncomfortable this conclusion might be, "progressives can't stick their head in the sand".

But the killer punch of his research is that diversity not only reduces social capital between ethnic groups but also within ethnic groups. Diversity leads not so much to bad race relations as to everyone becoming more isolated and less trustful. In the jargon, it kills off both the "bridging capital" between different groups and "bonding capital", which is the connections among people like yourself. Putnam calls it "hunkering down" as people withdraw from all kinds of connectedness in their community. And what follows is a long list of negative consequences, which include less confidence in local government and the media, lower voting registration (though higher participation in protest), less volunteering, fewer close friends, lower rates of happiness and perceived quality of life and more time spent watching television. It affects almost all our relationships, from the most public to the most intimate.

Putnam and his team are too rigorous for any of the usual objections to stick. To reach his conclusion, he controlled for a wide range of other factors including inequality, poverty, residential mobility and education, to be sure that "hunkering" was really a response to ethnic diversity. He wasn't going to publish these kinds of explosive findings without being pretty sure he was right.

What's still not clear to him is what causes the hunkering and whether social psychology might provide some answers. Certainly social psychologists are not unfamiliar with the phenomenon. A study of American schools after desegregation found that children were defining who they would play with more narrowly than ever - "resegregation" followed lines not only of ethnicity but also of gender.

What makes Putnam nervous now is how this could be seized upon by rightwing politicians hostile to immigration. So he insists his research be seen in the context a) that ethnic diversity is increasing in all modern societies and is not only inevitable but is also desirable, a proven asset in terms of creativity and economic growth; and b) that "hunkering" can be short term and "successful immigrant societies create new forms of social solidarity".

In conversation, he emphasises the latter, well aware that he is publishing his findings at a time of intense anxiety over these issues both in the US (where legislation to legalise some of the estimated 12 million illegal immigrants just got thrown out of Congress) and in Europe. He doesn't underestimate the scale of the challenge, particularly in European countries that, he acknowledges, "haven't been immigrant societies for 1,000 years". He says that the "major social learning process" required is in the same league as that required by the industrial revolution.

But as he arrives in Manchester at the start of a major comparative project of social capital between the UK and the US, his big theme is don't panic. He rattles through US history to offer all kinds of illustrations of how large-scale migration can be successfully accommodated in a bid to allay some of the European anxiety, particularly around its Muslim minorities.

Neither the US nor Europe is currently facing the kind of levels of migration relative to population seen at the turn of the 20th century in the US. To the argument that the shiploads arriving in Ellis Island were all Europeans who thus had some common culture, he points out that at the time there was a rich alarmist literature of how racially distinct the Jewish or Italian immigrant groups were. The US has had a history of "exceptionalism" - the line "that past immigration is fine, but current immigrants present an unprecedented problem" - yet each new wave in turn is absorbed as successfully as the last.

US history shows that all migrant groups develop an intense religiosity - Irish, Italian, Jewish, Hispanic. The increasing religious identification of Muslims in Europe fits neatly into a well-established pattern. As do the tendencies to marry within ethnic and faith communities, and to maintain close ties to the country of origin - none of these inhibit integration in the long term.

You could say that they are part of the pattern of settlement as the first couple of generations maintain a strong migrant identity - which is, paradoxically, an important part of their capacity to integrate. A strong community identity gives them the confidence and self-respect to establish themselves and get on.

The frequent UK response to the US experience is that it's not relevant here. The US has a civic nationalism which facilitates the melting pot - the flags and pledges of allegiance But in fact US civic nationalism was deliberately invented at the end of the 19th century in the US precisely to replace an ethno-nationalism challenged by mass immigration. The implication is quite clear: it's up to the UK to develop a comparable civic nationalism, a point that has not been lost on any of the protagonists in the UK debate to whom Putnam has been speaking, from Trevor Phillips to Ruth Kelly, as their frequent statements about British identity indicate. If you want to understand what's driving the political establishment, read Putnam.

The only problem is that they seem to give more prominence to some of his ideas than others. Too often the public debate is skewed towards getting "them" to integrate with "us", and conform to "our" norms of dress, culture and values. When this is allied to an aggressive rhetoric on the war against terror, it begins to sound like hectoring or some form of persecution. But Putnam is not talking about a top-down set of instructions on nationalism, but a much broader social process in which the host country changes as much as it, changes its new arrivals: through a collaborative effort of imagination and myriad individual experiences, new solidarity is forged. It's a message of hope that he keenly hopes doesn't get buried in sensationalist headlines about the short term cost of "hunkering".